The techniques described herein relate to the field of optical metrology. Optical methods for control of thin film properties in semiconductor (and other) device manufacturing environments have become widely accepted. Particular advantages of using optical metrology include a high measurement throughput and the fact that optical measurements are typically nondestructive.
The most common optical metrology techniques are reflectometry and ellipsometry. Ellipsometry is generally regarded as consisting of a “richer” dataset, including a measurement of two quantities per wavelength/incident angle. On the other hand, reflectometers are more robust due to less complex hardware configuration, have faster measurements, and typically have a smaller footprint. Generally speaking, if both technologies are capable of solving a given metrology problem, the reflectometer is a more cost effective choice for a high-volume production environment.
Semiconductor device manufacturing is characterized by continually decreasing feature sizes. For example, in integrated circuit (IC) devices, the shrinking of the gate length has caused a corresponding decrease in the gate dielectric thickness to the order of 1 nm. Consequently, an important manufacturing issue is control of properties of ultra-thin films such as for example silicon oxynitrides or hafnium silicate films. Usually, control of film thickness is of primary importance, but control of film composition can be equally important, since both properties influence the final IC device performance.
This shrinking of device dimensions is where vacuum ultra-violet wavelength metrology comes in. It is well-known that a decrease in incident wavelength enhances sensitivity of the detected signal to minute changes in samples properties. An example is reflectance of ˜1-2 nm silicon dioxide films on silicon substrates. FIGS. 1A and 1B compare simulated reflectances of 10 Å SiO2/Si film (plot 101), 11 Å SiO2/Si film (plot 102), and 12 Å SiO2/Si film (plot 103). Changes in film thickness are only detectable in the deep-ultra violet (DUV) and VUV regions, are more resolved the shorter the wavelength, and are undetectable in the visible wavelength regions. FIG. 1A shows a reflectance range of 30% to 80% and a wavelength range of 120 nm to 1000 nm, while FIG. 1B is an expanded version of a portion of FIG. 1A, with a reflectance range of 45% to 70% and a wavelength range of 120 nm to 220 nm. The differences between the reflectances of plots 101, 102 and 103 are more apparent in FIG. 1B.
Somewhat less known in the art is the ability to distinguish the effects of multiple parameters on the detected spectrum as the incident wavelength decreases below DUV regions. The ability to determine changes in film thickness and composition independently is enhanced in the VUV region, where many films exhibit very rich absorption spectra. Thus, using only DUV wavelengths, it may be possible to distinguish thickness or composition changes in an ultra-thin film, but not simultaneously. To do this with a reflectometer, one must move to VUV wavelengths, as illustrated in “Optical characterization of hafnium-based high-k dielectric films using vacuum ultraviolet reflectometry” (C. Rivas, XV International Conference on Vacuum Ultraviolet Radiation Physics, published 2007) for the case of HfxSi1-xO2, or in FIGS. 2A-C for silicon oxynitride (SiON). FIGS. 2A, 2B, and 2C compare reflectances for three SiON film cases: 30 Å thick, 15% nitride component (plot 201), 31 Å thick 15% nitride component (plot 202), and 30 Å thick, 17% nitride component (plot 203). FIG. 2A shows a reflectance range of 10% to 80%, and a wavelength range of 120 nm to 1000 nm. FIG. 2B shows an expanded version of a portion of FIG. 2A, with a reflectance range of 15% to 55%, and a wavelength range of 120 nm to 160 nm. FIG. 2C shows a second expanded version of a portion of FIG. 2A, with a reflectance range of 60% to 70%, and a wavelength of 180 nm to 300 nm. FIG. 2B shows that VUV reflectance can be used to distinguish all three films. FIG. 2C illustrates how DUV reflectance can distinguish the first film from the other two, but cannot distinguish the change of 1 Å thickness from a change of 2% nitride component. In addition, the variety and richness of absorption structure in the VUV for many dielectric materials means that reflectance data often contains as much as or even more information than ellipsometric data, even when the data is taken from the same wavelength region. FIG. 2D shows the optical parameters, n and k, for the oxide and nitride components of the oxynitride film. In FIG. 2D, n SiO2 plot 206, k SiO2 plot 207, n Si3N4 plot 208, and k Si3N4 plot 209 are shown. The large difference in absorption properties (as indicated in the k spectra) in the VUV regions is a key enabler for VUV reflectometery.
Consequently, a VUV reflectometer has been disclosed in U.S. Pat. Nos. 7,026,626, 7,067,818, 7,126,131, and 7,271,394, the disclosures of which are expressly incorporated herein by reference in their entirety. This reflectometer has overcome the difficulties involved with VUV operation, and in particular incorporates an inert gas environment, as well as a real-time reference procedure to enhance stability.
A formidable obstacle to stable, reliable metrology at VUV wavelengths is a buildup of contaminants on optical surfaces during operation. This contaminant buildup is generally characteristic of all optical systems operating in the VUV region, and has also been observed in initial 157 nm lithographic systems, as seen in “Contamination rates of optical surface at 157 nm in the presence of hydrocarbon impurities”, (T. M. Bloomstein, V. Liverman, M. Rothschild, S. T. Palmacci, D. E. Hardy, and J. H. C. Sedlacek, Optical Microlithography XV, Proceedings of the SPIE, Vol. 4691, p. 709, published 2002) and “Contamination monitoring and control on ASML MS-VII 157 nm exposure tool”, (U. Okoroanyanwu, R. Gronheid, J. Coenen, J. Hermans, K. Ronse, Optical Microlithography XVII, Proceedings of the SPIE, Vol. 5377, p. 1695, published 2004), as well as space-based VUV experiments, such as “Optical Characterization of Molecular Contaminant Films”, (Photonics Tech Briefs, January 2007). For fab production environments, the contaminant is thought to involve a photodeposition process as VUV light interacts with siloxanes, hydrocarbons, and other compounds common in fab environments.
One method for calibrating a VUV reflectometer system that takes into account contaminant buildup has been disclosed in U.S. patent application Ser. No. 10/930,339 filed on Aug. 31, 2004, Ser. No. 11/418,827 filed May 5, 2006 (now U.S. Pat. No. 7,282,703), Ser. No. 11/418,846 filed May 5, 2006, and Ser. No. 11/789,686, filed on Apr. 25, 2007, which are all expressly incorporated herein by reference in their entirety. This method involves using a reflectance ratio, which is independent of incident system intensity, to measure properties of contaminant layers on the calibration samples. The measured contaminant layer properties are used to calculate the reflectance spectra of the calibration samples, which enables the determination of the incident intensity from the intensity reflected from the calibration sample. Once the incident intensity is known, an absolute reflectance can be measured for any subsequent sample.